Significant behavioral and neuroendocrine effects of a variety of peripheral hormones including the gonadal steroids, as well as the pituitary and gastrointestinal peptides have been described by a number of investigators. Little data exist as to the neurochemical mechanisms involved in the interaction of these hormones with excitable tissues, including central neurotransmitter systems. Studies in our laboratory have been designed to elucidate the possible biochemical effects of hormones on neuronal preparations in vivo and in vitro. Significant effects of biologically active estrogens on catecholamine efflux or "release" from the isolated hypothalamus in vitro have been demonstrated. The conversion of estrogens to novel catechol metabolites (catechol estrogens) by the brain and other neuroendocrine tissues has also been reported. Recent studies on the gastrointestinal peptide cholecystokinin have been undertaken and the presence of specific receptors for CCK have been demonstrated in the CNS.